This invention relates to a method of producing acrylic resins useable in films and acrylic resins obtained by such method which is superior in transparency, toughness and has improved tendency to stress whiten without decrease of anti-weathering property.
Methyl methacrylate polymer is an useful synthetic resin, economically, since it has superior weathering properties and is used in various fields as formed products, such as prepared by casting, injection molding and extrusion molding. The main defect of this resin is its brittleness although it is hard. This has limited use of this resin. However, recently, various improved products having advanced toughness, much better than that of methyl methacrylate polymer itself, have been produced by, for example, methods employing the blending of an elastic component, grafting of methyl methacrylate to an elastic polymer or copolymerization of methyl methacrylate with a monomer which polymerizes to a soft polymer.
These products are now widely used in areas where materials are desired having such properties as softness, flexibility, after-workability and especially anti-weathering properties. For example, the products may be made either solely or mixed with other soft materials into films or sheets.
Among various methods for producing such materials, graft copolymerization is believed to be better than random copolymerization since the polymer obtained by graft copolymerization is superior to that obtained by random copolymerization from the standpoint of properties demanded for film or sheet, such as those above mentioned.
Further, in graft copolymerization, generally, a saturated elastomer is used in order not to decrease the characteristic antiweathering properties of the obtained graft polymer. The saturated elastomer may be a polymer of acrylic acid alkyl ester. There are various known methods for graft copolymerizing using the acrylic acid alkyl ester elastomer as the component to be grafted. Examples area: A method which introduces unsaturated group into the side chain of acrylic acid alkyl ester elastomer in order to improve the compatibility between the elastomer and the resin component which is obtained from the monomer mixture in the graft copolymerization. A method which adds methacrylic acid alkyl ester containing acrylic acid alkyl ester stepwisely into the reaction system in the presence of acrylic acid alkyl ester elastomer in order to obtain sufficient compatibility between the elastomer component and the resin component. A method which prepares a cross-linking structure in the molecule of acrylic acid alkyl ester elastomer as the resin reinforcing material. However, since the materials obtained by such method have a tendency to easily become stress whitened, their commercial value is decreased substantially.
For example, in the case where a graft polymer prepared by a conventional method exemplified above is used as a laminate on the surface of metallic plate or as material for building use, the polymer very often shows stress whitening phenomenon when subjected to secondary working, such as bending and cutting, at the position or about the position where the mechanical stress such as stretching, compression or impulse is applied. This, unfortunately, restricts the application and utility of the product. The stress whitening occurs by the appearance of crazes in the graft polymer when a stress is applied thereto. The applied stress is concentrated at the elastomer component whose modulus of elasticity is small. Thus, the refractive index of the part having such crazes decreases and a double refraction in the graft polymer occurs.
The crazes appear in the neighborhood of the interface between the elastic part and the resinous part of the graft polymer due to large differences of elastic modulus between the two or to poor binding between them. Accordingly, the occurrence of stress whitening can be delayed with a long induction period if the difference of elastic modulus is small or the binding force between the two parts is improved. Furthermore, it is certain that in case graft polymerization is carried out to graft a resinous component onto a saturated elastomer as a reinforcing material, since chemical bonding between them scarecely happens, many kinds of homopolymers co-exist. Accordingly, the interface between them is inhomogeneous and the obtained polymer has a tendency to become turbid. If stress is applied to such a polymer, it generates many crazes about the interfaces stressed concentratedly, that stress whitening is produced.